Dupliframes not rendered in Cycles?
Ok, got the answer.. it was a setting. 

By accident, in two files from different users, Duplication was set to 'Frames' not only on the Dupli object, but also on its parent curve. I guess it must be an easy slip to make.

I would delete the question, but I've referred to it from elsewhere, so I'll leave it for a bit before deleting it, and the comments which reference it.